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Handbook of air conditioning and refrigeration / Shan K

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CHAPTER 16<br />

AIR SYSTEMS: EQUIPMENT—<br />

AIR-HANDLING UNITS AND<br />

PACKAGED UNITS<br />

16.1 AIR-HANDLING UNITS 16.1<br />

Functions <strong>of</strong> Air-H<strong>and</strong>ling Units 16.1<br />

Classifications <strong>of</strong> Air-H<strong>and</strong>ling Units 16.2<br />

16.2 MAIN COMPONENTS 16.4<br />

Casing 16.4<br />

Fans 16.4<br />

Coils 16.5<br />

Filters 16.5<br />

Humidifiers 16.5<br />

Outdoor Air Intake, Mixing, <strong>and</strong> Exhaust<br />

Section 16.6<br />

Controls 16.6<br />

Component Layout 16.6<br />

Coil Face Velocity 16.8<br />

16.3 SELECTION OF AIR-HANDLING<br />

UNITS 16.9<br />

16.4 PACKAGED UNITS 16.12<br />

Types <strong>of</strong> Packaged Unit 16.12<br />

16.1 AIR-HANDLING UNITS<br />

Functions <strong>of</strong> Air-H<strong>and</strong>ling Units<br />

Ro<strong>of</strong>top Packaged Units 16.12<br />

Indoor Packaged Units 16.15<br />

Split Packaged Units 16.16<br />

16.5 PERFORMANCE AND SELECTION OF<br />

PACKAGED UNITS 16.17<br />

Indoor Environmental Control 16.17<br />

Indoor Air Quality 16.18<br />

Scroll Compressors <strong>and</strong> Evaporative<br />

Condensers 16.18<br />

Controls 16.18<br />

Minimum Performance 16.19<br />

Selection <strong>of</strong> Packaged Units 16.19<br />

16.6 FAN ROOM 16.24<br />

Types <strong>of</strong> Fan Room 16.24<br />

Layout Considerations 16.25<br />

REFERENCES 16.28<br />

An <strong>air</strong>-h<strong>and</strong>ling unit (AHU) is the primary equipment in an <strong>air</strong> system <strong>of</strong> a central hydronic system;<br />

it h<strong>and</strong>les <strong>and</strong> conditions the <strong>air</strong> <strong>and</strong> distributes it to various conditioned spaces. In an AHU, the<br />

required amounts <strong>of</strong> outdoor <strong>air</strong> <strong>and</strong> recirculating <strong>air</strong> are <strong>of</strong>ten mixed <strong>and</strong> conditioned. The temperature<br />

<strong>of</strong> the discharge <strong>air</strong> is then maintained within predetermined limits by means <strong>of</strong> control<br />

systems. After that, the conditioned supply <strong>air</strong> is provided with motive force <strong>and</strong> is distributed to<br />

various conditioned spaces through ductwork <strong>and</strong> space diffusion devices.<br />

Many <strong>air</strong>-h<strong>and</strong>ling units are modular so that they have the flexibility to add components as<br />

required. An AHU basically consists <strong>of</strong> an outdoor <strong>air</strong> intake <strong>and</strong> mixing box section, a fan section<br />

including a supply fan <strong>and</strong> a fan motor, a coil section with a water cooling coil, a filter section, <strong>and</strong><br />

a control section. A return or relief fan, a heating coil, a precooling coil, <strong>and</strong> a humidifier may also<br />

be included depending on the application. Supply volume flow rates <strong>of</strong> AHUs vary from 2000 to<br />

63,000 cfm (945 to 29,730 L/s).<br />

Whether a return fan or a relief fan should be added to an <strong>air</strong> system depends on the construction<br />

<strong>and</strong> operating characteristics <strong>of</strong> the <strong>air</strong> system <strong>and</strong> the total pressure loss <strong>of</strong> the return<br />

system (see Chap. 22). A heating coil is mainly used in the <strong>air</strong>-h<strong>and</strong>ling unit that serves the perimeter<br />

zone, or for morning warm-up in the heating season. The use <strong>of</strong> a precooling coil, to draw<br />

16.1

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